In packaging technique, laminated material of a complex build-up is used frequently so as to take advantage of the best properties of the different single materials and to combine them to give the optimum combination for the particular kind of package or the particular product which is to be packed. Thus it is known that paper, cardboard or rigid foamed plastic sheet are excellent materials for providing a package with a rigidity such that the material by folding can be formed into packing containers of lasting shape which provide a good mechanical protection for the goods enclosed. However, as is well known paper and cardboard are neither water nor grease resistant nor air-tight, so that the paper material must be combined with other material layers which have these desired qualities.
The laminates produced in this manner can be relatively complex in their build-up owing to a great number of layers being included in the same laminate. Consequently, they are expensive to manufacture, because the laminate has to be built up successively layer by layer. It is also known that plastic material can be coextruded so as to achieve both a better material adhesion and a plastic film containing two or more different plastic layers in a single manufacturing operation.
In the present case, it is desired to build up a packing laminate which has a carrier layer of paper, cardboard or for example polystyrene foam, which implies that packages manufactured from the laminate can be formed by folding to a lasting shape and that a good mechanical protection is imparted to the contents. Moreover, the laminate should be liquid-tight so as not to absorb moisture or liquid which may come into contact with the outside of the package and the package should have an inside which is liquid-tight and which can be heat-sealed by bringing together plastic layers which can be fused together with the help of heat and pressure to a mechanically strong union. Furthermore the inner plastic layer which is in direct contact with the contents should have a low characteristic taste level in cases where the contents are constituted of foodstuffs. The inner plastic film should be sufficiently strong and tough to withstand the stresses on the material occasioned by the fold-forming without causing the plastic material to split. Furthermore, the laminate should include a gas barrier, that is to say a layer which prevents the passage of gas, and as such a layer of aluminum foil can preferably be used. Thus a number of different material layers are to be joined to one another and this is done with the help of different binder layers.
The present invention is further concerned with the manufacturing method of laminated web material possessing a layer of aluminium foil and a layer of thermoplastic synthetic resin material on the inner and external surfaces, respectively, of the substrate. Conventionally, when manufacturing such a laminated web material (laminated paper), broadly speaking, there are used two methods.
The first method is called dry laminating method, in which after applying adhesive to laminated material (subsubstrate) such as a film made from synthetic resin and the like and drying the same, such a subsubstrate is pasted to the surface of the substrate or other laminated material.
Another method, as shown in FIG. 4, is called extrusion laminating method, in which a thin film of synthetic resin material, melted at the temperature as high as 300.degree. C., is injected from an extrusion die and a coated layer of synthetic resin material is formed on the surface of the aluminium foil by the injected synthetic resin material.
What is packaged by the laminated web material which is manufactured by the conventional method gives rise to unreasonably abnormal smell in the content or deteriorates its flavor, such an improper change being more frequently found, whenever it is packaged for longer preservation or at higher temperature.
That is because, for type manufactured by the dry laminating method, for example, there exists a layer of adhesive between the aluminium foil located on the inner surface of a packaging container and a layer of synthetic resin material and said adhesive is eluted.
On the other hand, the extrusion laminating method causes oxidative product (carboxyl group) in the synthetic resin material, by itself, forming a layer of synthetic resin material, the oxidative product is led to outbreak of the abnormal smell and deterioration of the flavor.
Namely, in order to form the packaging container to be filled with a drink, the container by itself is required to have sufficient strength as well as there is such a necessity that the synthetic resin material with which the inner surface of the aluminium foil is laminated is firmly adhered to the aluminium foil so that the container may not be deformed, a leakage of the drink to be packaged being prevented. In order to adhere firmly the synthetic resin material to the aluminium foil, however, in the case of polyethylene resin, for example, the synthetic resin material needs to be injected at a high temperature over 300.degree. C. The synthetic resin material which is injected at the high temperature sticks closely to the surface of the aluminium foil and gets in contact with the air for relatively longer time before forming a thin coated layer thereon, resulting in a great number of oxidative product. Conversely, if the synthetic resin material is maintained at a lower temperature so as to prevent the outbreak of the oxidative product, the adherence with the aluminum foil becomes weaker and the workability of the container becomes worse.
The oxidative product existing in the layer of syntethic resin material is gradually eluted into the goods to be packaged particularly in the situation of higher temperatures, wherein the degree of elution is increased.
For this reason, a package using the laminated web material manufactured by the conventional method cannot be readily heated so as to preserve food sensitive to taste and flavor for a long time.